1. Field of the Invention
This invention relates to a fuel cell and, more particularly, to a solid-state polymer fuel cell in which a polymer ion-exchange membrane is utilized as one of preferable electrolyte membranes.
2. Description of the Related Art
The conventional solid-state polymer fuel cell known in the art has a polymer ion-exchange membrane such as Nafion (Trademark of Dupont) and a pair of fuel and oxidation electrodes provided in a state confronting to each other through the polymer ion-exchange membrane so that the fuel cell fulfills its function as an electric power source in a vehicle when hydrogen and oxygen are supplied into the fuel and oxidation electrodes, respectively.
Such a solid-state polymer fuel cell has been expected to be one of preferable electric power sources for an electric motor used in the vehicle solely or in combination with an internal combustion engine such as a gasoline engine since it has achieved lightweighting and reduced space in total and the used electrolyte membrane always kept its solid state without loosing its quantity.
The Japanese Patent Application Laid-open No. Hei3-295176 teaches a typical conventional solid-state polymer fuel cell as depicted in FIG. 8. This fuel cell is generally defined by a solid-state polymer electrolyte membrane 1 and gas diffusion electrodes 2A and 2B fixed on the solid-state polymer electrolyte membrane 1 so as to confront to each other via the membrane 1. The gas diffusion electrode 2A as the fuel electrode is provided to receive therein hydrogen gas through a gas separator 5 and the other gas diffusion electrode 2B as the oxidation electrode is provided to receive therein oxygen gas through a gas separator 6.
Both gas separators 5 and 6 are generally constructed of a conductive thin plate (e.g., metal plate) preventing a mixing of the hydrogen and oxygen gases supplied to the fuel and oxidation electrodes respectively when plural cells are connected in series. As the gas separators are able to electrically relate to the fuel and oxidation electrodes in series, they are adapted to electrically connect one with the other when plural cells are connected in series.
The gas diffusion electrode 2A, 2B is defined by a reaction membrane 3A, 3B adjacent to the solid-state electrolyte membrane 1 and a gas diffusion membrane 4A, 4B adjacent to the gas separator 5, 6.
The gas diffusion membrane 4A, 4B is adapted to preferably diffuse the gas such as hydrogen or oxygen fed into the gas separator 5, 6 and transfer the diffused gas to the reaction membrane 3A, 3B. The advantages inherent in the gas diffusion membrane 4A, 4B are not to be corroded by sulfonic group (--SO.sub.3 H) as an ingredient of the solid-state polymer electrolyte membrane 1 but to have an electrical conductivity. Therefore, it has generally been considered that a sheet-state carbon was the most preferable material for constructing the gas diffusion membrane.
The reaction membrane 3A, 3B is expected to carry out a so-called battery reaction at an interface contacting with the solid-state polymer electrolyte membrane 1 when receiving the gas fed through the gas diffusion membrane 4A, 4B. Accordingly, a sintered mixture consisting of a water-repellent material such as polytetrafluorethylene (PTFE) and a catalyst particles such as platinum (Pt) or platinum supported on carbon black (Pt/C) has generally been used in the art.
However, the sheet-state carbon used for the gas diffusion membrane for the electrode in the above-mentioned conventional fuel cell unexpectedly shows a high electrical resistance.
The gas permeability of the sheet-state carbon just has a gas-supplying rate of 850 ml.multidot.mm/hr.multidot.cm.sup.2 .multidot.mmAq according to data shown in a catalog prepared for a generally known carbon paper.
In view of the above, when one considers to achieve an effective electricity generation in the one or plural fuel cells, the feature of the gas diffusion membrane, that is, to have a lower electrical resistance than the sheet-state carbon and a high gas permeability becomes particularly important.
Accordingly, the inventor of the present application had conceived to use, for the gas diffusion membrane or layer, a member made from porous metallic material such as nickel (Ni) foam in stead of the sheet-state carbon. Several examples conducted by the inventor based on the above analysis show that an electrode employing the gas diffusion membrane made from such a porous metallic material has a low electrical resistance and more effective diffusion of the gas fed therein compared with that of the sheet-state carbon.
On the contrary however, the nickel is easy to be corroded by sulfonic group contained in the electrolyte membrane to thereby dissolve into the electrolyte membrane unexpectedly. The inventor has therefore improved the conventional fuel cell as taught in the Japanese Patent Application No. Hei5-27362, wherein a corrosion resistant layer made from carbon is newly introduced between the reaction membrane or a catalyst layer and the gas diffusion membrane or a gas diffusion layer.
More particularly, in the above-mentioned improved fuel cell, the fuel cell comprised a solid-state electrolyte membrane and fuel and oxidation electrodes each of which has a gas diffusion layer made from porous metallic material to receive therein a hydrogen or oxygen gas, a catalyst layer contacting with the electrolyte membrane and, between the gas diffusion layer and the catalyst layer, a corrosion resistant layer functioning not to corrode the gas diffusion layer by strong acidic constituent contained in the electrolyte membrane.
The fuel cell according to the Japanese Patent Application No. Hei5-27362 may dissolve disadvantages recognized in the above-described conventional fuel cell, but it has still been required for the person in the art to reduce electrical resistance as much as possible when using.
Other objects, features and advantages of the present invention will become apparent upon reading the following specification and claims when taken in conjunction with the accompanying drawings.